In order to increase the efficiency of a power plant, especially in large power plants, a reheater is often used. In the reheater the steam that has expanded through a high-pressure turbine is superheated again in medium pressure. From the reheater the heated steam is directed to a medium pressure turbine.
From the point of view of operation it is necessary to be able to control the temperature of the steam to be reheated. For this purpose, various solutions have been developed. One known solution is a spraying cooler. In the solution in question water is sprayed to the reheating steam, in which case the temperature of the steam decreases. The solution is simple, but its problem is that it decreases the total efficiency of the plant. In addition, it is not always possible to use a spraying system.
Because of the problems of the spraying control, other control solutions have also been aimed to be developed. One solution is known from the publication WO 90/08917, which describes such a reheating structure of a fluidized bed boiler, which comprises a two-part reheater and a control structure of steam, by means of which a part of the steam to be reheated can be directed past the first reheater.